Design and Simulation of Single-Metal Semi-Insulating Surface Plasmon Waveguides for GaN-Based Terahertz Quantum Cascade Lasers

Abstract Waveguide design strategies of GaN-based terahertz quantum cascade lasers (THz QCLs) have been investigated. Single-metal semi-insulating surface plasmon waveguides for GaN THz QCLs have been simulated and analyzed. Optical fields can be effectively confined within the active region while maintaining low waveguide losses. Due to the compromise between confinement factor and waveguide losses, QCLs operating at different frequencies require distinct plasma layer (n++ layer) properties. The bulk plasma frequency, determined by electron density and characterized by high free-carrier absorption and low refractive index, must be detuned from the target QCL operational frequency. Increasing the doping concentration of the top contact layer (n+ layer) and the bottom confinement n++ layer shifts its plasma frequency away from the QCL operation frequency, and thereby achieving reduced threshold gain. Proper waveguide designs demonstrate threshold gains as low as 30-40 cm -1 , for the undeveloped 5-12 THz gap, which would be covered by a GaN THz QCL theoretically.